A transmission line type of the co-planar line has been well known for transmitting high frequency signals thereon. The co-planar line includes a signal line and ground lines or patterns sandwiching the signal line therebetween. When an additional ground metal is placed closer to the co-planar line, such an additional metal pattern possibly influences the transmission characteristic of the co-planar line, in particular, the transmission impedance thereof. Also, an apparatus implementing the co-planar line is requested to securely connect the ground lines to the ground.